The present invention is directed to a Wien filter used, for example, in a scanning electron microscope (xe2x80x9cSEMxe2x80x9d) and, in particular, to an improved design for minimizing aberrations that affect the primary electron beam to thereby improve instrument resolution.
Various instruments are known which rely on emission of charged particles from a sample to derive characteristics of the sample. Examples of such instruments are electron microscopes (e.g., scanning electron microscopes), focused ion beam microscopes, and mass spectrometers which utilize various well known means to analyze charged particles emitted from the sample.
For facilitating the description of the present invention, it will be explained in connection with an SEM. However, it should be understood that the invention is not limited to an SEM and can be applied by one with ordinary skill in the art to other instruments such as those mentioned above.
An SEM operates by generating a primary scanning electron beam that impacts a sample a surface of which is being imaged. As a result, backscattered and secondary electrons are emitted from the sample surface and have respective trajectories backward along the original beam direction which is perpendicular to the sample surface (known as the on-axis direction) and at angles diverging therefrom. Emitted electrons are collected by a detector, which is arranged above the sample. The detector generates a signal from the electron emission collected from the sample surface as it is exposed to the electron beam. The signal from the detector is typically processed to create an image of the surface, which is then displayed on a video screen.
With structures such as high aspect-ratio trenches and contact holes, the only electrons able to escape are those emitted on-axis. However, standard SEMs do not readily detect on-axis emitted electrons. This is because the detector must be spaced from the axis so as not to impede the primary, or incident, beam. In that position, on-axis electrons do not impinge upon the detector. Also, the high voltage on the front surface of the detector, which is commonly used to attract the secondary electrons, may adversely affect the primary beam.
The term xe2x80x9cfilterxe2x80x9d as used herein refers to devices used to in some way separate, or disperse, the particles of a charged particle beam through their inherent differences in either velocity (although often, in fact usually, this quantity is classified as xe2x80x9cenergyxe2x80x9d), charge, or mass. This can be done through application of either electric or magnetic fields, or a combination of both.
Wien filters have long been known (see W. Wien, Ann. Phys. 65 (1898), page 444). In such a filter, electrodes and magnetic poles are simultaneously utilized to create both an electric field and a magnetic field. The two fields are tuned, or adjusted, to apply equal and opposite forces to electrons in the incident beam, so that it is not deflected. However, an electron moving in the direction opposite to the incident beam will be oppositely affected by that same magnetic field force, which acts on such electron in the same direction as the electric field force. Thus, that electron will be deflected away from the beam axis. In that way, even the on-axis particles can be detected as they are deflected to a properly positioned detector.
U.S. Pat. No. 4,658,136 of Apr. 14, 1987, entitled xe2x80x9cSecondary Electronic Detecting Apparatusxe2x80x9d suggests the use of a Wien filter in a scanning electron microscope, but in practice Wien filters have not been used in that application because they have caused relatively small but significant disturbances in the electrostatic and magnetic fields through which the particles of the primary beam and the emitted particles pass, thereby disturbing the trajectories of such particles, which disturbances have been sufficient to undesirably degrade the primary beam and hence the resolution of the SEM. More specifically, the use of a Wien filter in an SEM requires that the magnetic and electrostatic fields be precisely matched and uniform in order to ensure that the primary electron beam is unperturbed and aberrations are kept to a minimum.
One object of the present invention is to provide a filter that generates electrical and magnetic fields which is constructed so as to significantly minimize aberrations that are due to mismatching of the electric and the magnetic fields, so that when the filter is used in combination with an SEM or the like, resolution of the instrument is not degraded to any appreciable degree.
Another object is to provide an external magnetic and electric field clamp, located adjacent to (above and below) the pole pieces to further improve the field shapes.
A further object of the present invention is to so construct a filter that its parts can be readily assembled to produce a precision structure which is physically and electrically stable under operating conditions.
Another object of the present invention is to provide a filter that has electric and magnetic fields which are uniform and precisely matched, thereby to effectively and accurately function in an SEM or the like.
Another object is to place the connectors to the poles and coils outside of the vacuum to facilitate manufacturing and eliminate outgassing from the coils.
These and other objects are attained by one aspect of the present invention directed to an improvement in an electromagnetic filter comprising a field-producing structure at least partly surrounding a passage. The improvement comprises a supporting structure around the passage having a plurality of inwardly extending openings therethrough. Each of a plurality of magnetically permeable field-producing structures extends continuously through one of the openings, terminating at one end in a pole face located radially inwardly from the supporting structure and constituted at its other end by a portion extending radially outwardly from the supporting structure. Each of the field-producing structures engages the supporting structure. Electromagnetic field-producing elements are operatively connected to the radially outwardly extending portions of the field-producing structures and are effective to act on the field-producing structures to produce magnetic and/or electric fields emanating therefrom and extending into the passage. The filter is adapted to function in combination with magnetic circuit means located outside the supporting structure for operatively engaging and coupling the radially outwardly extending portions of the field-producing structures with a gap between the field-producing structures and the magnetic circuit means, the gap being located radially outside the supporting structure.
In accordance with the present invention, a novel arrangement of the component parts of the filter is provided. In particular, the gap in the magnetic circuit which is required for electrical isolation is properly located so that the fringe magnetic fields associated with it, which otherwise tend to perturb the fields acting on the particles passing through the filter, do not create any deleterious effects on resolution.
We have discovered that locating the radial gap in the magnetic circuit near the central axis of the instrument produces undesirable disturbances in the magnetic field, which in turn adversely affect the resolution of the instrument. More specifically, a higher and more uniform and more stable magnetic field results when the radial gap is located laterally outside the structure which supports the pole pieces as far as possible from the axis of the beam and, even more preferably, outside the windings mounted on the pole pieces. These improvements in function are achieved by a structure which is comparatively simple, sturdier and more reliable than prior art Wien filter structures.
As indicated above, while the filter of the present invention is here specifically described for use with a scanning electron microscope, in connection with which its improved function is exceedingly important, it will be understood that the utility of the filter under discussion is not limited to that application, but is also advantageous in connection with most instruments where charged particles move through the same space in different directions and are to be differently acted upon depending upon the particular direction in which they are moving.